Enhanced Charge Carrier Separation in WO3/BiVO4 Photoanodes Achieved via Light Absorption in the BiVO4 Layer

Photoelectrochemical (PEC) water splitting converts solar light and water into oxygen and energy-rich hydrogen. WO3/BiVO4 heterojunction photoanodes perform much better than the separate oxide components, though internal charge recombination undermines their PEC performance when both oxides absorb light. Here we exploit the BiVO4 layer to sensitize WO3 to visible light and shield it from direct photoexcitation to overcome this efficiency loss. PEC experiments and ultrafast transient absorption spectroscopy performed by frontside (through BiVO4) or backside (through WO3) irradiating photoanodes with different BiVO4 layer thickness demonstrate that irradiation through BiVO4 is beneficial for charge separation. Optimized electrodes irradiated through BiVO4 show 40% higher photocurrent density compared to backside irradiation.

B ismuth vanadate, BiVO 4 , is a promising semiconductor oxide employed in photoanodes for the oxygen evolution reaction in water-splitting devices. 1,2 Its stability in contact with aqueous electrolytes, 3,4 its good visible light-harvesting capability, 5 and its simple preparation through cheap wet techniques 6 point to this material as a possible component of future commercial photoelectrochemical (PEC) cells. Furthermore, in the last 15 years the efficiency of BiVO 4 -based photoanodes (in terms of current density) rapidly grew from a few microamps per square centimeter in early reports to 4−5 mA cm −2 , with prolonged continuous operation of photoelectrodes modified with oxygen evolution cocatalysts. 3,4,6−13 However, the fast charge recombination of BiVO 4 -based electrodes still hampers the efficiency of this material. 2,14 A way to overcome this intrinsic flaw is to couple BiVO 4 with WO 3 in the WO 3 /BiVO 4 heterojunction where visible light harvesting BiVO 4 sensitizes wider band gap WO 3 . 15 BiVO 4 photoanodes based on this heterojunction achieve the highest current densities among oxide-based photoanodes. 16,17 The suitable band gap alignment between the two oxides, the efficient electron and hole transport in WO 3 and BiVO 4 , respectively, and the spacial charge separation support the high performance of WO 3 /BiVO 4 photoanodes. 18 −25 In previous studies, we investigated the charge carrier dynamics in the WO 3 /BiVO 4 system through transient absorption spectroscopy (TAS) with detection either in the visible range to observe the hole dynamics in BiVO 4 18,26−28 or in the mid-infrared to follow the electron dynamics in WO 3 and BiVO 4 . 29 We also identified wavelength-dependent processes by tuning the excitation wavelength across the WO 3 absorption edge (ca. 450 nm). 18,26,29 Indeed, the type II band alignment between the two oxides ( Figure 1A) allows distinct charge transfer processes leading to charge separation or recombination, depending on the excitation wavelength. Under visible light excitation of BiVO 4 , electrons promoted in its conduction band (CB) flow into the energetically lowerlying CB of WO 3 , while holes remain in the BiVO 4 valence band (VB). This electron transfer process (process Ⓐ in Figure  1A) decreases charge recombination and leads to long-living charge carriers that are beneficial for PEC performance. 29 Conversely, irradiation at wavelengths below 450 nm leads to the excitation of both oxides and opens a detrimental recombination path between the electrons photopromoted in the CB of WO 3 and the holes in BiVO 4 (process Ⓑ in Figure  1A). This process results in charge recombination on a ∼200 ps time scale 26 and becomes more relevant with increasing WO 3 layer thickness. 30 Based on these dynamics, we posited that an efficient heterojunction system needs to direct charges along process Ⓐ and disfavor off-track routes such as process Ⓑ. Still, solar light includes photons energetic enough to excite WO 3 (∼4% of the solar spectrum is at wavelengths below the absorption edge of bulk WO 3 ). Therefore, a portion of photogenerated charges in WO 3 /BiVO 4 systems may be wasted through process Ⓑ. On the other hand, BiVO 4 efficiently absorbs light beyond the absorption edge of WO 3 , up to 520 nm, allowing us to exploit a larger fraction of the solar spectrum. Therefore, in this work we pursue the strategy of using the BiVO 4 sensitizer to shield WO 3 from direct photoexcitation.
We assembled a series of heterojunction electrodes with a WO 3 scaffold layer of fixed thickness (ca. 150 nm) coated with BiVO 4 overlayers with different thickness (15−160 nm) to tune the amount of light absorbed by BiVO 4 . First, a systematic PEC study allowed us to probe whether the irradiation mode (through WO 3 or BiVO 4 , backside or frontside irradiation, respectively, Figure 1B) affects the overall PEC efficiency of the electrodes. Then, transient absorption spectroscopy (TAS) with a pump in the UV region (387 nm) and detection in the visible range was employed to assess the effects of the irradiation mode on the lifetime of photogenerated holes in BiVO 4 . These tests allowed us to evaluate the extent of charge recombination induced by process Ⓑ and its impact on the PEC performance of the heterojunction photoanodes as a function of the BiVO 4 layer thickness.
The WO 3 /BiVO 4 photoanodes were prepared through spin coating using fluorine-doped tin oxide (FTO) as the conductive glass substrate (see the Supporting Information). The heterojunction electrode with the thickest BiVO 4 layer almost entirely absorbs 387 nm photons, the pump wavelength in TAS experiments, Figure 1C. A series of control photoanodes consisting of pure BiVO 4 on FTO (without WO 3 layer) with variable BiVO 4 thickness was also prepared. The absorption spectra of the two electrode series are shown in Figures S1 and S2; the thickness of the BiVO 4 layer was estimated using the absorption coefficient at 420 nm, 19 α 40 = 6.7 × 10 4 cm −1 . XRD analyses confirm the successful synthesis of WO 3 and BiVO 4 ( Figure S3) and FESEM images  demonstrate the uniform coating of the photoanodes ( Figure  S4).
In order to explore the shielding hypothesis, we carried out PEC experiments on the electrodes. Figure 2A, B shows the photocurrent density generated with the WO 3 /BiVO 4 electrodes under simulated solar light irradiation in 0.5 M Na 2 SO 4 solution under back-and frontside irradiation at different applied potentials. The linear sweep voltammetry plots are reported in Figures S5 and S6. As a general trend, all heterojunction photoanodes outperform control pure BiVO 4 electrodes (see Figure S7). Furthermore, the better light exploitation achieved with increasing the BiVO 4 layer thickness drives the photocurrent increase under both irradiation conditions up to a 75 nm thick BiVO 4 layer.
Under frontside simulated solar light irradiation ( Figure  2A), the heterojunction photoanodes generate considerably higher photocurrent than in backside mode ( Figure 2B). The best performing electrode with a 75 nm BiVO 4 layer thickness ( Figure 2C), when irradiated frontside shows a ca. 40% increase in the current density, from 1.0 to 1.38 mA cm −2 , with respect to backside irradiation, at the formal H 2 O/O 2 redox potential of 1.23 V vs the standard hydrogen electrode (V SHE ).
We used single-wavelength efficiency measurements to gather further information on this PEC performance increase. Specifically, internal quantum efficiency (IQE, Figure 3 and Figures S8 and S9), measuring the efficiency of absorbed photons, was calculated from the incident photon to current efficiency (IPCE, see Figures S10 and S11) recorded with the WO 3 /BiVO 4 electrodes in contact with a 0.5 M Na 2 SO 4 solution at 1.23 V SHE . Figure 3A, B shows the IQE vs BiVO 4 thickness contour plots measured under frontside and backside irradiation.
Under frontside irradiation, the IQE reaches the highest values for 70−130 nm thick BiVO 4 layers in the WO 3 /BiVO 4 heterojunction, as evidenced by the red/yellow island appearing in Figure 3A as opposed to the green plot obtained under backside irradiation, which leads to lower IQE values ( Figure 3B). Notably, the largest IQE enhancement under frontside irradiation occurs below 450 nm, where WO 3 absorbs light, and for BiVO 4 layers thicker than 50 nm, which absorb a substantial fraction of incident light. The IQE in this irradiation mode maintains above 30% up to 450 nm for the best-performing electrodes, while it is seldom above 25% under backside irradiation, see for example the IQE traces for the WO 3 /BiVO 4 electrode with 75 nm thick BiVO 4 in Figure 3C.
At the same time, the IQE curves are similar under the two irradiation modes in the 450−520 nm range because only BiVO 4 absorbs light at these wavelengths and process Ⓐ ( Figure 1A) is predominantly active. Thus, these wavelengthdependent PEC analyses indicate that the performance of the WO 3 /BiVO 4 photoanodes benefits from avoiding WO 3 excitation. This condition occurs by selectively exciting BiVO 4 under frontside irradiation, i.e., by shielding WO 3 with BiVO 4 , and in both irradiation modes under excitation at wavelengths above the WO 3 absorption onset ( Figure 3C).
We then investigated the effect of WO 3 shielding on the lifetime of photogenerated holes in the BiVO 4 layer of the WO 3 /BiVO 4 system. Previous work ascribed the transient absorption ΔA signal at 470 nm to trapped holes in BiVO 4 , based on experiments in the presence of hole scavengers. 18,21,31 TAS proved an essential tool for studying charge carrier dynamics and diffusion in photocatalysis and photovoltaics. 32−35 Therefore, TAS with detection at 470 nm was here employed to investigate the dynamics of photogenerated holes in both pure BiVO 4 and WO 3 /BiVO 4 electrode series upon backside and frontside excitation at 387 nm.
The ΔA signals recorded with pure BiVO 4 electrodes were analyzed first. For this system, similar transient dynamics were obtained in the two irradiation modes. Figure S12 reports representative transient absorption spectra, while Figure S13 shows the transient decay ΔA profiles at 470 nm, which were fitted according to a biexponential decay model (eq 1).
In this equation, τ 1 and τ 2 are the lifetimes of the faster and slower decay processes typical of BiVO 4 , respectively, A 1 and A 2 are the weighted coefficients that represent the contribution of each of the two processes to the overall decay and ΔA 0 is the offset (set at zero in the fitting). 21 The fitting parameters for the BiVO 4 electrodes (Table S1) are in line with literature

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www.acsaem.org Letter reports on pure BiVO 4 . Regardless of the BiVO 4 thickness , A 1 and A 2 account of ∼30 and 70% of the hole decay, respectively. The fast decay lifetime, which is associated with the recombination of trapped holes in BiVO 4 with photopromoted free electrons, is independent of the BiVO 4 layer thickness (τ 1 , ∼20 ps), because all electrodes are excited at the same pump wavelength (i.e., with the same energy excess with respect to the BiVO 4 CB). 21,29 On the other hand, τ 2 , which is ascribed to the recombination of trapped holes with trapped electrons, increases from ∼1 to 6.5 ns with increasing BiVO 4 layer thickness as more holes get trapped in bulk sites. 28,36,37 The decay signal of photoproduced holes in the BiVO 4 layer of the WO 3 /BiVO 4 electrodes series recorded at 470 nm under backside and frontside irradiation are reported in Figure 4 and Figures S14 and S15. Under frontside excitation, the ΔA signals decay slower than under backside excitation ( Figure  4A−C). Indeed, under backside irradiation mode a significant fraction of 387 nm photons is absorbed by WO 3, leading to photoexcitation of electrons into its CB (the individual WO 3 layer absorbs ca. 16% of 387 photons, Figure 1C). Therefore, many photoproduced charge carriers recombine through process Ⓑ ( Figure 1A). This additional recombination channel leads to the abrupt ΔA drop observed during the first 400 ps following backside photoexcitation ( Figure 4A). Furthermore, under frontside irradiation the ΔA signal recorded with the WO 3 /BiVO 4 heterojunctions becomes progressively slower and comparable with those recorded with pure BiVO 4 ( Figure  4D−F and Figure S13).
In previous studies on the WO 3 /BiVO 4 heterojunction, we evaluated the contribution of process Ⓑ to the overall ΔA decay signal by fitting the ΔA decay traces including an additional decay component in eq 1, to take into account also process Ⓑ. 26 Here, we used the same approach to assess its contribution to the charge carrier dynamics in the two irradiation modes and fitted the ΔA decay with eq 2.
where τ r accounts for the additional recombination process and A r is its weighted contribution. We first fitted the dynamics recorded in the WO 3 /BiVO 4 electrodes under backside excitation. The fitting parameters are reported in Table 1. In this configuration, the WO 3 layer is

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www.acsaem.org Letter irradiated directly and absorbs the same amount of light in all electrodes. Therefore, we expect that process Ⓑ has a comparable effect on the charge carrier dynamics at each BiVO 4 layer thickness. Indeed, this process accounts for ca. 23 ± 8% of the holes decay, with a time constant τ r of ∼200 ps (Table 1). Under frontside irradiation, the BiVO 4 layer in the heterojunction electrodes shields WO 3 from light absorption as the BiVO 4 layer thickness increases. Indeed, the percent amount of incident 387 nm photons absorbed by the WO 3 underlayer in the coupled system progressively decreases ( Figure 1C). Therefore, we sought to quantify the shielding effect of the BiVO 4 layer in decreasing the extent of process Ⓑ in the WO 3 /BiVO 4 electrodes. By assuming that process Ⓑ operates with its intrinsic time constant τ r regardless of the excitation mode, we fitted the decay dynamics recorded under frontside irradiation by employing the τ r previously extracted from the TAS signals recorded upon excitation in backside mode ( Table 1). Because of the reduced amount of charge carriers generated in WO 3 , the weight of process Ⓑ in terms of the A r parameter (Table 2) decreases with increasing the BiVO 4 layer thickness. Additionally, as fewer charge carriers undergo process Ⓑ, A 2 increases, suggesting that a larger number of photogenerated charge carriers recombines through the slower process. A 160 nm thick BiVO 4 layer almost entirely absorbs the pump ( Figure 1C), preventing WO 3 excitation. Due to the lack of photoexcited electrons in the CB of WO 3 , the electrons photopromoted in the BiVO 4 CB can only recombine with trapped holes in BiVO 4 , or flow into WO 3 CB via process Ⓐ, resulting in better charge carrier separation. Consequently, the holes photogenerated in the BiVO 4 layer of the WO 3 /BiVO 4 heterojunction live longer than those in the individual 160 nm thick BiVO 4 electrode ( Figure 4F). This condition is akin to selective BiVO 4 excitation in WO 3 /BiVO 4 at wavelengths beyond WO 3 absorption edge, which we previously observed extending the hole lifetimes compared to individual BiVO 4 . 26,29 Thus, TAS and PEC experiments suggest that light absorption by the BiVO 4 layer in WO 3 /BiVO 4 electrodes selectively suppresses process Ⓑ and promotes process Ⓐ, which leads to an increase of trapped hole lifetime in BiVO 4 . However, despite the promise of long-living holes in the heterojunction electrode with a 160 nm thick BiVO 4 layer, it performs poorly compared to the most active heterojunction with the 75 nm thick BiVO 4 layer. This latter electrode possesses an optimal matching between (i) WO 3 sensitization to the visible light, (ii) photogenerated charge separation at the heterojunction, and (iii) efficient charge extraction toward the external circuit and the electrolyte. Indeed, thinner BiVO 4 layers limit the electrode performance due to the low visible light absorption, while thicker films may suffer from a greater charge recombination owing to hole accumulation in the BiVO 4 bulk under operando conditions.
In conclusion, we identified a shielding strategy to suppress the internal charge recombination occurring in the WO 3 / BiVO 4 heterojunction due to WO 3 excitation. Optimized light absorption in BiVO 4 layers considerably suppresses this recombination channel. The best performing electrode tested in this work shows a 40% increase in the PEC performance under frontside irradiation compared to backside irradiation. Furthermore, these findings suggest that methods to suppress undesired wavelength-dependent recombination processes and optimize charge transport and surface catalysis are required to design efficient photoelectrodes based on type-II heterojunctions. Complete contact information is available at: